Adjuvants by definition are substances that are incorporated into or are injected simultaneously with an antigen. The adjuvants potentiate nonspecifically the ensuing immune response. A principal purpose for employment of an immunotherapeutic adjuvant is to achieve more durable humoral or cell-mediated immunity of a high level by employing lower levels of an antigen with fewer numbers of doses than could be achieved by administering the equivalent aqueous antigen. Adjuvants are used in combination with non-living agents (in place of living microorganisms), for the preparation of vaccines. Adjuvants may also increase the effective immune response against low or nonimmunogenic tumor cells or cells infected with intracellular agents that are already present in the body and are not adequately checked by naturally elicited immune responses.
As studies slowly unravel the cellular and molecular mechanisms responsible for host immune responses, the challenge to design new compounds necessary to modulate both humoral and cell-mediated immunity becomes increasingly apparent. While hypotheses have been proposed as to how adjuvants augment the immune response, no substantial evidence exists which supports any particular theory or provides for a predictive rationale which can serve as a basis for designing improved adjuvants. To that extent then, while one can extrapolate from adjuvants which have been shown to be successful, there is still substantial uncertainty as to whether there will be success with new compositions.
With the advent of genetic engineering, there is now the possibility to develop any protein molecule, which can be used to mimic an epitope of an antigen of interest. For the most part, proteins by themselves do not elicit strong immune responses, as compared to a cell containing such protein. There is, therefore, an increased interest in being able to develop adjuvants which will potentiate immune responses to proteins or other antigenic compositions, e.g. saccharide or hapten conjugates, so as to produce neutralizing antibodies or cell-mediated immunity effective for protecting a host against a pathogen.